1. Field of the Invention
The present invention relates to an image heating apparatus suitable for use as a heat-fixing apparatus mounted on an image forming apparatus, such as a copier or printer. In particular, it relates to an image heating apparatus having heating means which contacts an outer peripheral surface of a “roller contacting a toner image”.
2. Related Background Art
Of image heat-fixing apparatus to be mounted on image forming apparatus, those of a heat roller type and a film heating type are in practical use.
Typically, the heat roller type involves pressing a fixing roller (heat roller) containing a heat source, such as a halogen lamp, against a pressure roller to form a nip portion that feeds a recording material. The heat source in the fixing roller is controlled based on the temperature detected by a temperature detecting element provided on an outer peripheral surface of the fixing roller. The heat-fixing apparatus of the film heating type comprise a film-like rotatable member (referred to as a film, hereinafter), a heater which contacts the inner surface of the film and comprises a heat-generating resistor on a substrate of ceramic or the like, and a pressure roller pressed against the heater with the film interposed therebetween, in which a recording material is fed between the film and the pressure roller to heat the toner image thereon. Such fixing apparatus have a temperature detecting element for detecting the temperature of the heater, and the heat generation by the heater is controlled based on the temperature detected by the temperature detecting element.
As described above, typically, in such apparatus, the turn-on of the heater is controlled to apply an adequate quantity of heat to the recording material.
In the case of the heat roller type, the temperature of the surface of the heat roller is typically controlled by measuring the temperature of the surface with temperature measuring means, such as a thermistor, which is brought into contact with the surface.
In the case of the film heating type, the temperature of the heater is controlled by detecting the temperature with a thermistor abutting against the heater.
However, the fixing apparatus of the heat roller type have a problem that the thermistor abutting against the fixing roller causes a scratch on the surface of the roller or a soil or the like on the thermistor impairs the image. To avoid the problem, the thermistor may be disposed out of the sheet-feeding area of the fixing roller. However, the temperature of the sheet-feeding area of the fixing roller is difficult to control because it cannot be directly detected. A temperature detecting element which does not contact the fixing roller may be used. However, the tradeoff is a higher cost. Besides, the fixing apparatus of the heat roller type have a basic problem that they have a high thermal capacity and require a long time to be activated to an operable state. On the other hand, in the fixing apparatus of the film heating type, since the thermistor is disposed in contact with the heater, it does not cause a scratch on the film surface. In addition, the fixing apparatus of the film heating type have a low thermal capacity, and, thus, require a shorter time to be activated to an operable state. However, if the film has no resilient layer, it is difficult to ensure a sufficient nip width and to heat the toner in a wrap-around manner. Therefore, they are not suitable for fixing a full-color toner image, although they are suitable for fixing a monochrome toner image. Besides, if the film has a resilient layer, there arises a problem that, because of the heat insulation by the resilient layer, the heater reaches its target temperature and stops generating heat before a sufficient quantity of heat is transferred to the film surface, so that the nip portion is not heated to a temperature suitable for fixing.
Thus, there has been devised a method of heating the fixing roller from the outside thereof (externally heating type) (eg. Japanese Patent Application Laid-Open No. H10-133505). According to this method, since heat can be supplied to the roller from the outside thereof, the temperature of the roller is highly responsive to the turn-on of the heater, and, thus, the activation time can be shortened.
However, if the temperature detecting means abuts against the roller surface, this method also has a problem of a scratch, soil or the like as in the heat roller type. In addition, the temperature of the fixing apparatus is difficult to control, and a disadvantage, such as hot offset of a toner image or fixability reduction, may occur.
Thus, there is a need for an image heating apparatus that has a short activation time and an adequate fixability for a full-color toner image and is insusceptible to scratch on the surface of a rotatable member contacting a toner image.